Solid electrolytic capacitor with improved leakage current

ABSTRACT

A capacitor assembly that is capable of exhibiting good electrical properties even under a variety of conditions is provided. More particularly, the capacitor contains a capacitor element that includes a sintered porous anode body, a dielectric that overlies the anode body, and a pre-coat layer that overlies the dielectric and is formed from an organometallic compound. A solid electrolyte overlies the pre-coat layer that contains an inner layer and an outer layer, wherein the inner layer is formed from an in situ-polymerized conductive polymer and the outer layer is formed from pre-polymerized conductive polymer particles.

BACKGROUND OF THE INVENTION

Solid electrolytic capacitors (e.g., tantalum capacitors) are typicallymade by pressing a metal powder (e.g., tantalum) around a metal leadwire, sintering the pressed part, anodizing the sintered anode, andthereafter applying a solid electrolyte. Intrinsically conductivepolymers are often employed as the solid electrolyte due to theiradvantageous low equivalent series resistance (“ESR”) and“non-burning/non-ignition” failure mode. For example, such electrolytescan be formed through in situ chemical polymerization of a3,4-dioxythiophene monomer (“EDOT”) in the presence of a catalyst anddopant. However, conventional capacitors that employ in situ-polymerizedpolymers tend to have a relatively high leakage current (“DCL”) and failat high voltages, such as experienced during a fast switch on oroperational current spike. In an attempt to overcome these issues,dispersions have also been employed that are formed from a complex ofpoly(3,4-ethylenedioxythiophene) and poly(styrene sulfonic acid(“PEDOT:PSS”). While the use of PEDOT:PSS dispersions can help lowerleakage current values, products containing such materials tend toexhibit a slow decrease in DCL over time after a voltage is applied tothe capacitor.

As such, a need currently exists for a solid electrolytic capacitorhaving an improved performance.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a capacitorassembly is disclosed that comprises a capacitor element. The capacitorelement comprises a sintered porous anode body, a dielectric thatoverlies the anode body, a pre-coat layer that overlies the dielectricand is formed from an organometallic compound, and a solid electrolytethat overlies the pre-coat layer. The solid electrolyte contains aninner layer and an outer layer, wherein the inner layer is formed froman in situ-polymerized conductive polymer and the outer layer is formedfrom pre-polymerized conductive polymer particles.

In accordance with another embodiment of the present invention, a methodfor forming a capacitor assembly is disclosed that comprises anodicallyoxidizing a sintered porous anode body to form an anode; applying asolution to the anode that includes an organometallic compound;polymerizing a conductive polymer precursor monomer in the presence ofan oxidative catalyst to form an inner solid electrolyte layer thatoverlies the pre-coat layer; and applying a dispersion of conductivepolymer particles to form an outer solid electrolyte layer that overliesthe inner solid electrolyte layer. Further, the organometallic compoundhas the following general formula:

wherein,

M is an organometallic atom;

R₁, R₂, and R₃ are independently an alkyl or a hydroxyalkyl, wherein atleast one of R₁, R₂, and R₃ is a hydroxyalkyl;

n is an integer from 0 to 8; and

X is an organic or inorganic functional group.

Other features and aspects of the present invention are set forth ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth more particularly in the remainder of the specification, whichmakes reference to the appended figures in which:

FIG. 1 is a cross-sectional view of one embodiment of a capacitorassembly of the assembly of the present invention;

FIG. 2 is a cross-sectional view of another embodiment of a capacitorassembly of the assembly of the present invention;

FIG. 3 is a cross-sectional view of yet another embodiment of acapacitor assembly of the assembly of the present invention; and

FIG. 4 is a top view of still another embodiment of a capacitor assemblyof the assembly of the present invention.

DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentinvention, which broader aspects are embodied in the exemplaryconstruction.

Generally speaking, the present invention is directed to a capacitorassembly that is capable of exhibiting good electrical properties undera wide variety of different conditions. More particularly, the capacitorassembly contains a capacitor element that includes a sintered porousanode body and a dielectric that overlies the anode body. The capacitorelement also contains a pre-coat layer that overlies the dielectric andincludes an organometallic compound. A solid electrolyte overlies thepre-coat layer that contains an inner layer formed from an insitu-polymerized conductive polymer and an outer layer formed frompre-polymerized conductive polymer particles. The present inventors havediscovered that the combination of the specific types of pre-coat layer,inner solid electrolyte layer, and outer solid electrolyte layer canresult in a capacitor assembly having a unique and beneficial array ofproperties. For example, after being subjected to an applied voltage(e.g., rated voltage) for 90 seconds, the capacitor assembly may exhibita DCL of only about 50 microamps (“μA”) or less, in some embodimentsabout 40 μA or less, in some embodiments about 30 μA or less, and insome embodiments, from about 0.1 to about 20 μA. Further, the rate ofDCL decay is relatively fast such that after being subjected to thevoltage for a total of 180 seconds, the capacitor assembly may exhibiteven lower DCL values, such as about greater than about 50%, in someembodiments greater than about 70%, and in some embodiments, greaterthan 80% of the DCL value after 90 seconds. For example, after 180seconds, the capacitor assembly may exhibit a DCL value of about 20 μAor less, in some embodiments about 10 μA or less, in some embodimentsabout 5 μA or less, and in some embodiments, from about 0.01 to about 5μA. Notably, the capacitor assembly may exhibit such low DCL valuesunder various conditions. For example, the capacitor assembly mayexhibit such low DCL values even after being placed into contact with anatmosphere having a high temperature, such as about 80° C. or more, insome embodiments from about 100° C. to about 150° C., and in someembodiments, from about 105° C. to about 130° C. (e.g., 105° C. or 125°C.). The capacitor assembly may also exhibit such DCL values when placedinto contact with an atmosphere having a relative humidity of about 10%or less, in some embodiments about 5% or less, and in some embodiments,from about 0.001% to about 1%. The dry atmosphere may be part of theinternal atmosphere of the capacitor assembly itself, or it may be anexternal atmosphere to which the capacitor assembly is exposed duringstorage and/or use.

Various embodiments of the invention will now be described in moredetail.

I. Capacitor Element

A. Anode Body

The capacitor element includes an anode that contains a dielectricformed on a sintered porous body. The porous anode body may be formedfrom a powder that contains a valve metal (i.e., metal that is capableof oxidation) or valve metal-based compound, such as tantalum, niobium,aluminum, hafnium, titanium, alloys thereof, oxides thereof, nitridesthereof, and so forth. The powder is typically formed from a reductionprocess in which a tantalum salt (e.g., potassium fluotantalate (K₂TaF),sodium fluotantalate (Na₂TaF₇), tantalum pentachloride (TaCl₅), etc.) isreacted with a reducing agent. The reducing agent may be provided in theform of a liquid, gas (e.g., hydrogen), or solid, such as a metal (e.g.,sodium), metal alloy, or metal salt. In one embodiment, for instance, atantalum salt (e.g., TaCl₅) may be heated at a temperature of from about900° C. to about 2,000° C., in some embodiments from about 1,000° C. toabout 1,800° C., and in some embodiments, from about 1,100° C. to about1,600° C., to form a vapor that can be reduced in the presence of agaseous reducing agent (e.g., hydrogen). Additional details of such areduction reaction may be described in WO 2014/199480 to Maeshima, etal. After the reduction, the product may be cooled, crushed, and washedto form a powder.

The specific charge of the powder typically varies from about 2,000 toabout 800,000 microFarads*Volts per gram (“μF*V/g”) depending on thedesired application. For instance, in certain embodiments, a high chargepowder may be employed that has a specific charge of from about 100,000to about 800,000 μF*V/g, in some embodiments from about 120,000 to about700,000 μF*V/g, and in some embodiments, from about 150,000 to about600,000 μF*V/g. In other embodiments, a low charge powder may beemployed that has a specific charge of from about 2,000 to about 100,000μF*V/g, in some embodiments from about 5,000 to about 80,000 μF*V/g, andin some embodiments, from about 10,000 to about 70,000 μF*V/g. As isknown in the art, the specific charge may be determined by multiplyingcapacitance by the anodizing voltage employed, and then dividing thisproduct by the weight of the anodized electrode body.

The powder may be a free-flowing, finely divided powder that containsprimary particles. The primary particles of the powder generally have amedian size (D50) of from about 5 to about 250 nanometers, in someembodiments from about 10 to about 200 nanometers, and in someembodiments, from about 20 to about 150 nanometers, such as determinedusing a laser particle size distribution analyzer made by BECKMANCOULTER Corporation (e.g., LS-230), optionally after subjecting theparticles to an ultrasonic wave vibration of 70 seconds. The primaryparticles typically have a three-dimensional granular shape (e.g.,nodular or angular). Such particles typically have a relatively low“aspect ratio”, which is the average diameter or width of the particlesdivided by the average thickness (“D/T”). For example, the aspect ratioof the particles may be about 4 or less, in some embodiments about 3 orless, and in some embodiments, from about 1 to about 2. In addition toprimary particles, the powder may also contain other types of particles,such as secondary particles formed by aggregating (or agglomerating) theprimary particles. Such secondary particles may have a median size (D50)of from about 1 to about 500 micrometers, and in some embodiments, fromabout 10 to about 250 micrometers.

Agglomeration of the particles may occur by heating the particles and/orthrough the use of a binder. For example, agglomeration may occur at atemperature of from about 0° C. to about 40° C., in some embodimentsfrom about 5° C. to about 35° C., and in some embodiments, from about15° C. to about 30° C. Suitable binders may likewise include, forinstance, poly(vinyl butyral); poly(vinyl acetate); poly(vinyl alcohol);poly(vinyl pyrollidone); cellulosic polymers, such ascarboxymethylcellulose, methyl cellulose, ethyl cellulose, hydroxyethylcellulose, and methylhydroxyethyl cellulose; atactic polypropylene,polyethylene; polyethylene glycol (e.g., Carbowax from Dow ChemicalCo.); polystyrene, poly(butadiene/styrene); polyamides, polyimides, andpolyacrylamides, high molecular weight polyethers; copolymers ofethylene oxide and propylene oxide; fluoropolymers, such aspolytetrafluoroethylene, polyvinylidene fluoride, and fluoro-olefincopolymers; acrylic polymers, such as sodium polyacrylate, poly(loweralkyl acrylates), poly(lower alkyl methacrylates) and copolymers oflower alkyl acrylates and methacrylates; and fatty acids and waxes, suchas stearic and other soapy fatty acids, vegetable wax, microwaxes(purified paraffins), etc.

The resulting powder may be compacted to form a pellet using anyconventional powder press device. For example, a press mold may beemployed that is a single station compaction press containing a die andone or multiple punches. Alternatively, anvil-type compaction pressmolds may be used that use only a die and single lower punch. Singlestation compaction press molds are available in several basic types,such as cam, toggle/knuckle and eccentric/crank presses with varyingcapabilities, such as single action, double action, floating die,movable platen, opposed ram, screw, impact, hot pressing, coining orsizing. The powder may be compacted around an anode lead, which may bein the form of a wire, sheet, etc. The lead may extend in a longitudinaldirection from the anode body and may be formed from any electricallyconductive material, such as tantalum, niobium, aluminum, hafnium,titanium, etc., as well as electrically conductive oxides and/ornitrides of thereof. Connection of the lead may also be accomplishedusing other known techniques, such as by welding the lead to the body orembedding it within the anode body during formation (e.g., prior tocompaction and/or sintering).

Any binder may be removed after pressing by heating the pellet undervacuum at a certain temperature (e.g., from about 150° C. to about 500°C.) for several minutes. Alternatively, the binder may also be removedby contacting the pellet with an aqueous solution, such as described inU.S. Pat. No. 6,197,252 to Bishop, et al. Thereafter, the pellet issintered to form a porous, integral mass. The pellet is typicallysintered at a temperature of from about 700° C. to about 1600° C., insome embodiments from about 800° C. to about 1500° C., and in someembodiments, from about 900° C. to about 1200° C., for a time of fromabout 5 minutes to about 100 minutes, and in some embodiments, fromabout 8 minutes to about 15 minutes. This may occur in one or moresteps. If desired, sintering may occur in an atmosphere that limits thetransfer of oxygen atoms to the anode. For example, sintering may occurin a reducing atmosphere, such as in a vacuum, inert gas, hydrogen, etc.The reducing atmosphere may be at a pressure of from about 10 Torr toabout 2000 Torr, in some embodiments from about 100 Torr to about 1000Torr, and in some embodiments, from about 100 Torr to about 930 Torr.Mixtures of hydrogen and other gases (e.g., argon or nitrogen) may alsobe employed.

B. Dielectric

The anode is also coated with a dielectric. The dielectric may be formedby anodically oxidizing (“anodizing”) the sintered anode so that adielectric layer is formed over and/or within the anode. For example, atantalum (Ta) anode may be anodized to tantalum pentoxide (Ta₂O₅).Typically, anodization is performed by initially applying a solution tothe anode, such as by dipping anode into the electrolyte. A solvent isgenerally employed, such as water (e.g., deionized water). To enhanceionic conductivity, a compound may be employed that is capable ofdissociating in the solvent to form ions. Examples of such compoundsinclude, for instance, acids, such as described below with respect tothe electrolyte. For example, an acid (e.g., phosphoric acid) mayconstitute from about 0.01 wt. % to about 5 wt. %, in some embodimentsfrom about 0.05 wt. % to about 0.8 wt. %, and in some embodiments, fromabout 0.1 wt. % to about 0.5 wt. % of the anodizing solution. Ifdesired, blends of acids may also be employed.

A current is passed through the anodizing solution to form thedielectric layer. The value of the formation voltage manages thethickness of the dielectric layer. For example, the power supply may beinitially set up at a galvanostatic mode until the required voltage isreached. Thereafter, the power supply may be switched to apotentiostatic mode to ensure that the desired dielectric thickness isformed over the entire surface of the anode. Of course, other knownmethods may also be employed, such as pulse or step potentiostaticmethods. The voltage at which anodic oxidation occurs typically rangesfrom about 4 to about 250 V, and in some embodiments, from about 9 toabout 200 V, and in some embodiments, from about 20 to about 150 V.During oxidation, the anodizing solution can be kept at an elevatedtemperature, such as about 30° C. or more, in some embodiments fromabout 40° C. to about 200° C., and in some embodiments, from about 50°C. to about 100° C. Anodic oxidation can also be done at ambienttemperature or lower. The resulting dielectric layer may be formed on asurface of the anode and within its pores.

Although not required, in certain embodiments, the dielectric layer maypossess a differential thickness throughout the anode in that itpossesses a first portion that overlies an external surface of the anodeand a second portion that overlies an interior surface of the anode. Insuch embodiments, the first portion is selectively formed so that itsthickness is greater than that of the second portion. It should beunderstood, however, that the thickness of the dielectric layer need notbe uniform within a particular region. Certain portions of thedielectric layer adjacent to the external surface may, for example,actually be thinner than certain portions of the layer at the interiorsurface, and vice versa. Nevertheless, the dielectric layer may beformed such that at least a portion of the layer at the external surfacehas a greater thickness than at least a portion at the interior surface.Although the exact difference in these thicknesses may vary depending onthe particular application, the ratio of the thickness of the firstportion to the thickness of the second portion is typically from about1.2 to about 40, in some embodiments from about 1.5 to about 25, and insome embodiments, from about 2 to about 20.

To form a dielectric layer having a differential thickness, amulti-stage process is generally employed. In each stage of the process,the sintered anode is anodically oxidized (“anodized”) to form adielectric layer (e.g., tantalum pentoxide). During the first stage ofanodization, a relatively small forming voltage is typically employed toensure that the desired dielectric thickness is achieved for the innerregion, such as forming voltages ranging from about 1 to about 90 volts,in some embodiments from about 2 to about 50 volts, and in someembodiments, from about 5 to about 20 volts. Thereafter, the sinteredbody may then be anodically oxidized in a second stage of the process toincrease the thickness of the dielectric to the desired level. This isgenerally accomplished by anodizing in an electrolyte at a highervoltage than employed during the first stage, such as at formingvoltages ranging from about 50 to about 350 volts, in some embodimentsfrom about 60 to about 300 volts, and in some embodiments, from about 70to about 200 volts. During the first and/or second stages, theelectrolyte may be kept at a temperature within the range of from about15° C. to about 95° C., in some embodiments from about 20° C. to about90° C., and in some embodiments, from about 25° C. to about 85° C.

The electrolytes employed during the first and second stages of theanodization process may be the same or different. Typically, however, itis desired to employ different solutions to help better facilitate theattainment of a higher thickness at the outer portions of the dielectriclayer. For example, it may be desired that the electrolyte employed inthe second stage has a lower ionic conductivity than the electrolyteemployed in the first stage to prevent a significant amount of oxidefilm from forming on the internal surface of anode. In this regard, theelectrolyte employed during the first stage may contain an acidiccompound, such as hydrochloric acid, nitric acid, sulfuric acid,phosphoric acid, polyphosphoric acid, boric acid, boronic acid, etc.Such an electrolyte may have an electrical conductivity of from about0.1 to about 100 mS/cm, in some embodiments from about 0.2 to about 20mS/cm, and in some embodiments, from about 1 to about 10 mS/cm,determined at a temperature of 25° C. The electrolyte employed duringthe second stage typically contains a salt of a weak acid so that thehydronium ion concentration increases in the pores as a result of chargepassage therein. Ion transport or diffusion is such that the weak acidanion moves into the pores as necessary to balance the electricalcharges. As a result, the concentration of the principal conductingspecies (hydronium ion) is reduced in the establishment of equilibriumbetween the hydronium ion, acid anion, and undissociated acid, thusforms a poorer-conducting species. The reduction in the concentration ofthe conducting species results in a relatively high voltage drop in theelectrolyte, which hinders further anodization in the interior while athicker oxide layer, is being built up on the outside to a higherformation voltage in the region of continued high conductivity. Suitableweak acid salts may include, for instance, ammonium or alkali metalsalts (e.g., sodium, potassium, etc.) of boric acid, boronic acid,acetic acid, oxalic acid, lactic acid, adipic acid, etc. Particularlysuitable salts include sodium tetraborate and ammonium pentaborate. Suchelectrolytes typically have an electrical conductivity of from about 0.1to about 20 mS/cm, in some embodiments from about 0.5 to about 10 mS/cm,and in some embodiments, from about 1 to about 5 mS/cm, determined at atemperature of 25° C.

If desired, each stage of anodization may be repeated for one or morecycles to achieve the desired dielectric thickness. Furthermore, theanode may also be rinsed or washed with another solvent (e.g., water)after the first and/or second stages to remove the electrolyte.

C. Pre-Coat Layer

As indicated above, a pre-coat layer overlies the dielectric thatincludes an organometallic compound. The organometallic compound mayhave the following general formula:

wherein,

M is an organometallic atom, such as silicon, titanium, and so forth;

R₁, R₂, and R₃ are independently an alkyl (e.g., methyl, ethyl, propyl,etc.) or a hydroxyalkyl (e.g., hydroxymethyl, hydroxyethyl,hydroxypropyl, etc.), wherein at least one of R₁, R₂, and R₃ is ahydroxyalkyl;

n is an integer from 0 to 8, in some embodiments from 1 to 6, and insome embodiments, from 2 to 4 (e.g., 3); and

X is an organic or inorganic functional group, such as glycidyl,glycidyloxy, mercapto, amino, vinyl, etc.

In certain embodiments, R₁, R₂, and R₃ may a hydroxyalkyl (e.g., OCH₃).In other embodiments, however, R₁ may be an alkyl (e.g., CH₃) and R₂ andR₃ may a hydroxyalkyl (e.g., OCH₃).

Further, in certain embodiments, M may be silicon so that theorganometallic compound is an organosilane compound, such as analkoxysilane. Suitable alkoxysilanes may include, for instance,3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane,3-aminopropylmethyldimethoxysilane, 3-aminopropylmethyldiethoxysilane,3-(2-aminoethyl)aminopropyltrimethoxysilane,3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane,3-mercaptopropylmethyldimethoxysilane,3-mercaptopropylmethyldiethoxysilane, glycidoxymethyltrimethoxysilane,glycidoxymethyltriethoxysilane, glycidoxymethyl-tripropoxysilane,glycidoxymethyltributoxysilane, β-glycidoxyethyltrimethoxysilane,β-glycidoxyethyltriethoxysilane, β-glycidoxyethyl-tripropoxysilane,β-glycidoxyethyl-tributoxysilane, β-glycidoxyethyltrimethoxysilane,α-glycidoxyethyltriethoxysilane, α-glycidoxyethyltripropoxysilane,α-glycidoxyethyltributoxysilane, γ-glycidoxypropyl-trimethoxysilane,γ-glycidoxypropyltriethoxysilane, γ-glycidoxypropyl-tripropoxysilane,γ-glycidoxypropyltributoxysilane, β-glycidoxypropyltrimethoxysilane,β-glycidoxypropyl-triethoxysilane, β-glycidoxypropyltripropoxysilane,α-glycidoxypropyltributoxysilane, α-glycidoxypropyltrimethoxysilane,α-glycidoxypropyltriethoxysilane, α-glycidoxypropyl-tripropoxysilane,α-glycidoxypropyltributoxysilane, γ-glycidoxybutyltrimethoxysilane,δ-glycidoxybutyltriethoxysilane, δ-glycidoxybutyltripropoxysilane,δ-glycidoxybutyl-tributoxysilane, δ-glycidoxybutyltrimethoxysilane,γ-glycidoxybutyltriethoxysilane, γ-glycidoxybutyltripropoxysilane,γ-propoxybutyltributoxysilane, δ-glycidoxybutyl-trimethoxysilane,δ-glycidoxybutyltriethoxysilane, δ-glycidoxybutyltripropoxysilane,α-glycidoxybutyltrimethoxysilane, α-glycidoxybutyltriethoxysilane,α-glycidoxybutyl-tripropoxysilane, α-glycidoxybutyltributoxysilane,(3,4-epoxycyclohexyl)-methyl-trimethoxysilane,(3,4-epoxycyclohexyl)methyl-triethoxysilane,(3,4-epoxycyclohexyl)methyltripropoxysilane,(3,4-epoxycyclohexyl)-methyl-tributoxysilane,(3,4-epoxycyclohexyl)ethyl-trimethoxysilane,(3,4-epoxycyclohexyl)ethyl-triethoxysilane,(3,4-epoxycyclohexyl)ethyltripropoxysilane,(3,4-epoxycyclohexyl)ethyltributoxysilane,(3,4-epoxycyclohexyl)propyltrimethoxysilane,(3,4-epoxycyclohexyl)propyltriethoxysilane,(3,4-epoxycyclohexyl)propyl-tripropoxysilane,(3,4-epoxycyclohexyl)propyltributoxysilane,(3,4-epoxycyclohexyl)butyltrimethoxysilane, (3,4-epoxycyclohexy)butyltriethoxysilane, (3,4-epoxycyclohexyl)butyltripropoxysilane,(3,4-epoxycyclohexyl)butyltributoxysilane, and so forth.

The particular manner in which the pre-coat layer is applied to thecapacitor body may vary as desired. In one particular embodiment, thecompound is dissolved in an organic solvent and applied to the part as asolution, such as by screen-printing, dipping, electrophoretic coating,spraying, etc. The organic solvent may vary, but is typically analcohol, such as methanol, ethanol, etc. Organometallic compounds mayconstitute from about 0.1 wt. % to about 10 wt. %, in some embodimentsfrom about 0.2 wt. % to about 8 wt. %, and in some embodiments, fromabout 0.5 wt. % to about 5 wt. % of the solution. Solvents may likewiseconstitute from about 90 wt. % to about 99.9 wt. %, in some embodimentsfrom about 92 wt. % to about 99.8 wt. %, and in some embodiments, fromabout 95 wt. % to about 99.5 wt. % of the solution. Once applied, thepart may then be dried to remove the solvent therefrom and form apre-coat layer containing the organometallic compound.

D. Solid Electrolyte

As indicated above, a solid electrolyte overlies the pre-coat layer andgenerally functions as the cathode for the capacitor assembly. The solidelectrolyte contains one or more inner layers and one or more outerlayers that overly the inner layer(s). The term “inner” in this contextrefers to one or more layers that overly the pre-coat layer, whetherdirectly or via another layer (e.g., adhesive layer). Likewise, the term“outer” refers to one or more layers that overly the inner layer(s) andare formed from a different material.

i. Inner Layer

As indicated, the solid electrolyte contains one or more “inner” layersthat contain a conductive polymer that is in situ-polymerized over thepre-coat layer. For example, such in situ-polymerized conductivepolymers may constitute about 50 wt. % or more, in some embodimentsabout 70 wt. % or more, and in some embodiments, about 90 wt. % or more(e.g., 100 wt. %) of the inner layer(s). One or multiple inner layersmay be employed. For example, the solid electrolyte may contain from 2to 30, in some embodiments from 4 to 20, and in some embodiments, fromabout 5 to 15 inner layers (e.g., 10 layers). Regardless, the conductivepolymer is typically r-conjugated and has electrical conductivity afteroxidation or reduction, such as an electrical conductivity of at leastabout 1 μS/cm. Examples of such r-conjugated conductive polymersinclude, for instance, polyheterocycles (e.g., polypyrroles,polythiophenes, polyanilines, etc.), polyacetylenes, poly-p-phenylenes,polyphenolates, and so forth. In one embodiment, for example, thepolymer is a substituted polythiophene having repeating units of thefollowing general formula (I):

wherein,

T is O or S;

D is an optionally substituted C₁ to C₅ alkylene radical (e.g.,methylene, ethylene, n-propylene, n-butylene, n-pentylene, etc.);

R₇ is a linear or branched, C₁ to C₁₈ alkyl radical (e.g., methyl,ethyl, n- or iso-propyl, n-, iso-, sec- or tert-butyl, n-pentyl,1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 1-ethylpropyl,1,1-dimethylpropyl, 1,2-dimethylpropyl, 2,2-dimethylpropyl, n-hexyl,n-heptyl, n-octyl, 2-ethylhexyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl,n-tridecyl, n-tetradecyl, n-hexadecyl, n-octadecyl, etc.); C₅ to C₁₂cycloalkyl radical (e.g., cyclopentyl, cyclohexyl, cycloheptyl,cyclooctyl, cyclononyl, cyclodecyl, etc.); C to C₁₄ aryl radical (e.g.,phenyl, naphthyl, etc.); C₇ to C₁₈ aralkyl radical (e.g., benzyl, o-,m-, p-tolyl, 2,3-, 2,4-, 2,5-, 2-6, 3-4-, 3,5-xylyl, mesityl, etc.); and

q is an integer from 0 to 8, in some embodiments, from 0 to 2, and inone embodiment, 0.

Example of substituents for the radicals “D” or “R₇” include, forinstance, alkyl, cycloalkyl, aryl, aralkyl, alkoxy, halogen, ether,thioether, disulphide, sulfoxide, sulfone, sulfonate, amino, aldehyde,keto, carboxylic acid ester, carboxylic acid, carbonate, carboxylate,cyano, alkylsilane and alkoxysilane groups, carboxylamide groups, and soforth.

Particularly suitable thiophene polymers are those in which “D” is anoptionally substituted C₂ to C₃ alkylene radical. For instance, thepolymer may be optionally substituted poly(3,4-ethylenedioxythiophene),which has repeating units of the following general formula (II):

Methods for forming conductive polymers, such as described above, arewell known in the art. For instance, U.S. Pat. No. 6,987,663 to Merker,et al., describes various techniques for forming substitutedpolythiophenes from a thiophene precursor monomer. In one particularembodiment, “q” is 0. One commercially suitable example of3,4-ethylenedioxthiophene is available from Heraeus under thedesignation Clevios™ M. Other suitable monomers are also described inU.S. Pat. No. 5,111,327 to Blohm, et al. and U.S. Pat. No. 6,635,729 toGroenendaal, et al. Derivatives of these monomers may also be employedthat are, for example, dimers or trimers of the above monomers. Highermolecular derivatives, i.e., tetramers, pentamers, etc. of the monomersare suitable for use in the present invention. The derivatives may bemade up of identical or different monomer units and used in pure formand in a mixture with one another and/or with the monomers. Oxidized orreduced forms of these precursors may also be employed.

To form an in situ-polymerized layer, the precursor monomer may bepolymerized in the presence of an oxidative catalyst (e.g., chemicallypolymerized). The oxidative catalyst typically includes a transitionmetal cation, such as iron(III), copper(II), chromium(VI), cerium(IV),manganese(IV), manganese(VII), or ruthenium(III) cations, and etc. Adopant may also be employed to provide excess charge to the conductivepolymer and stabilize the conductivity of the polymer. The dopanttypically includes an inorganic or organic anion, such as an ion of asulfonic acid. In certain embodiments, the oxidative catalyst has both acatalytic and doping functionality in that it includes a cation (e.g.,transition metal) and an anion (e.g., sulfonic acid). For example, theoxidative catalyst may be a transition metal salt that includesiron(III) cations, such as iron(III) halides (e.g., FeCl₃) or iron(III)salts of other inorganic acids, such as Fe(ClO₄)₃ or Fe₂(SO₄)₃ and theiron(III) salts of organic acids and inorganic acids comprising organicradicals. Examples of iron (III) salts of inorganic acids with organicradicals include, for instance, iron(III) salts of sulfuric acidmonoesters of C₁ to C₂₀ alkanols (e.g., iron(III) salt of laurylsulfate). Likewise, examples of iron(III) salts of organic acidsinclude, for instance, iron(III) salts of C₁ to C₂₀ alkane sulfonicacids (e.g., methane, ethane, propane, butane, or dodecane sulfonicacid); iron (III) salts of aliphatic perfluorosulfonic acids (e.g.,trifluoromethane sulfonic acid, perfluorobutane sulfonic acid, orperfluorooctane sulfonic acid); iron (III) salts of aliphatic C₁ to C₂₀carboxylic acids (e.g., 2-ethylhexylcarboxylic acid); iron (III) saltsof aliphatic perfluorocarboxylic acids (e.g., trifluoroacetic acid orperfluorooctane acid); iron (III) salts of aromatic sulfonic acidsoptionally substituted by C₁ to C₂₀ alkyl groups (e.g., benzene sulfonicacid, o-toluene sulfonic acid, p-toluene sulfonic acid, ordodecylbenzene sulfonic acid); iron (III) salts of cycloalkane sulfonicacids (e.g., camphor sulfonic acid); and so forth. Mixtures of theseabove-mentioned iron(III) salts may also be used. Iron(III)-p-toluenesulfonate, iron(III)-o-toluene sulfonate, and mixtures thereof, areparticularly suitable. One commercially suitable example ofiron(III)-p-toluene sulfonate is available from Heraeus under thedesignation Clevios™ C.

The oxidative catalyst and precursor monomer may be applied eithersequentially or together to initiate the polymerization reaction.Suitable application techniques for applying these components includescreen-printing, dipping, electrophoretic coating, and spraying. As anexample, the monomer may initially be mixed with the oxidative catalystto form a precursor solution. Once the mixture is formed, it may beapplied to the anode and then allowed to polymerize so that a conductivecoating is formed on the surface. Alternatively, the oxidative catalystand monomer may be applied sequentially. In one embodiment, for example,the oxidative catalyst is dissolved in an organic solvent (e.g.,butanol) and then applied as a dipping solution. The part may then bedried to remove the solvent therefrom. Thereafter, the part may bedipped into a solution containing the monomer. Regardless,polymerization is typically performed at temperatures of from about −10°C. to about 250° C., and in some embodiments, from about 0° C. to about200° C., depending on the oxidizing agent used and desired reactiontime. Suitable polymerization techniques, such as described above, maybe described in more detail in U.S. Pat. No. 7,515,396 to Biler. Stillother methods for applying such conductive coating(s) may be describedin U.S. Pat. No. 5,457,862 to Sakata, et al., U.S. Pat. No. 5,473,503 toSakata, et al., U.S. Pat. No. 5,729,428 to Sakata, et al., and U.S. Pat.No. 5,812,367 to Kudoh, et al.

ii. Outer Layer

As noted above, the solid electrolyte also contains one or more “outer”layers that are formed from a different material than the inner layer(s)and overly the inner layer(s). More particularly, the outer layer(s) areformed from pre-polymerized intrinsically and/or extrinsicallyconductive polymer particles. One benefit of employing such particles isthat they can minimize the presence of ionic species (e.g., Fe²⁺ orFe³⁺) produced during conventional in situ polymerization processes,which can cause dielectric breakdown under high electric field due toionic migration. Thus, by applying the conductive polymer aspre-polymerized particles rather through in situ polymerization, theresulting capacitor may exhibit a relatively high “breakdown voltage.”In one particular embodiment, for example, the outer layer(s) are formedprimarily from such conductive polymer particles in that they constituteabout 50 wt. % or more, in some embodiments about 70 wt. % or more, andin some embodiments, about 90 wt. % or more (e.g., 100 wt. %) of arespective outer layer. One or multiple outer layers may be employed.For example, the solid electrolyte may contain from 2 to 30, in someembodiments from 3 to 25, and in some embodiments, from about 4 to 20outer layers, each of which may optionally be formed from a dispersionof the pre-polymerized conductive polymer particles. Regardless of thenumber of layers employed, the resulting solid electrolyte, includingall of the inner layer(s) and outer layer(s), typically has a total athickness of from about 1 micrometer (μm) to about 200 μm, in someembodiments from about 2 μm to about 50 μm, and in some embodiments,from about 3 μm to about 30 μm.

As indicated above, the conductive polymer particles of the outerlayer(s) may be formed from an extrinsically and/or intrinsicallyconductive polymer. In certain embodiments, an “extrinsically”conductive polymer may be employed in the solid electrolyte that hasrepeating units of the formula (I) or (II) above. Such polymers aregenerally considered to be “extrinsically” conductive to the extent thatthey typically require the presence of a separate counterion that is notcovalently bound to the polymer. The counterion may be a monomeric orpolymeric anion that counteracts the charge of the conductive polymer.Polymeric anions can, for example, be anions of polymeric carboxylicacids (e.g., polyacrylic acids, polymethacrylic acid, polymaleic acids,etc.); polymeric sulfonic acids (e.g., polystyrene sulfonic acids(“PSS”), polyvinyl sulfonic acids, etc.); and so forth. The acids mayalso be copolymers, such as copolymers of vinyl carboxylic and vinylsulfonic acids with other polymerizable monomers, such as acrylic acidesters and styrene. Likewise, suitable monomeric anions include, forexample, anions of C₁ to C₂₀ alkane sulfonic acids (e.g., dodecanesulfonic acid); aliphatic perfluorosulfonic acids (e.g.,trifluoromethane sulfonic acid, perfluorobutane sulfonic acid orperfluorooctane sulfonic acid); aliphatic C₁ to C₂₀ carboxylic acids(e.g., 2-ethyl-hexylcarboxylic acid); aliphatic perfluorocarboxylicacids (e.g., trifluoroacetic acid or perfluorooctanoic acid); aromaticsulfonic acids optionally substituted by C₁ to C₂₀ alkyl groups (e.g.,benzene sulfonic acid, o-toluene sulfonic acid, p-toluene sulfonic acidor dodecylbenzene sulfonic acid); cycloalkane sulfonic acids (e.g.,camphor sulfonic acid or tetrafluoroborates, hexafluorophosphates,perchlorates, hexafluoroantimonates, hexafluoroarsenates orhexachloroantimonates); and so forth. Particularly suitablecounteranions are polymeric anions, such as a polymeric carboxylic orsulfonic acid (e.g., polystyrene sulfonic acid (“PSS”)). The molecularweight of such polymeric anions typically ranges from about 1,000 toabout 2,000,000, and in some embodiments, from about 2,000 to about500,000.

Intrinsically conductive polymers may also be employed that have apositive charge located on the main chain that is at least partiallycompensated by anions covalently bound to the polymer. For example, oneexample of a suitable intrinsically conductive polymer may haverepeating units of the following formula (III):

wherein,

R is (CH₂)_(a)—O—(CH₂)_(b);

a is from 0 to 10, in some embodiments from 0 to 6, and in someembodiments, from 1 to 4 (e.g., 1);

b is from 1 to 18, in some embodiments from 1 to 10, and in someembodiments, from 2 to 6 (e.g., 2, 3, 4, or 5);

Z is an anion, such as SO₃ ⁻, C(O)O⁻, BF₄ ⁻, CF₃SO₃ ⁻, SbF₆ ⁻,N(SO₂CF₃)₂ ⁻, C₄H₃O₄ ⁻, ClO₄ ⁻, etc.;

X is a cation, such as hydrogen, an alkali metal (e.g., lithium, sodium,rubidium, cesium or potassium), ammonium, etc.

In one particular embodiment, Z in formula (I) is a sulfonate ion suchthat the intrinsically conductive polymer contains repeating units ofthe following formula (IV):

wherein, R and X are defined above. In formula (III) or (IV), a ispreferably 1 and b is preferably 3 or 4. Likewise, X is preferablysodium or potassium.

If desired, the polymer may be a copolymer that contains other types ofrepeating units. In such embodiments, the repeating units of formula(III) typically constitute about 50 mol. % or more, in some embodimentsfrom about 75 mol. % to about 99 mol. %, and in some embodiments, fromabout 85 mol. % to about 95 mol. % of the total amount of repeatingunits in the copolymer. Of course, the polymer may also be a homopolymerto the extent that it contains 100 mol. % of the repeating units offormula (III). Specific examples of such homopolymers includepoly(4-(2,3-dihydrothieno-[3,4-b][1,4]dioxin-2-ylmethoxy)-1-butane-sulphonicacid, salt) andpoly(4-(2,3-dihydrothieno-[3,4-b][1,4]dioxin-2-ylmethoxy)-I-propanesulphonicacid, salt).

Regardless of the particular nature of the polymer, the conductivepolymer particles used to form the outer layer(s) typically have anaverage size (e.g., diameter) of from about 1 to about 80 nanometers, insome embodiments from about 2 to about 70 nanometers, and in someembodiments, from about 3 to about 60 nanometers. The diameter of theparticles may be determined using known techniques, such as byultracentrifuge, laser diffraction, etc. The shape of the particles maylikewise vary. In one particular embodiment, for instance, the particlesare spherical in shape. However, it should be understood that othershapes are also contemplated by the present invention, such as plates,rods, discs, bars, tubes, irregular shapes, etc.

Although not necessarily required, the conductive polymer particles maybe applied in the form of a dispersion. The concentration of theconductive polymer in the dispersion may vary depending on the desiredviscosity of the dispersion and the particular manner in which thedispersion is to be applied to the capacitor element. Typically,however, the polymer constitutes from about 0.1 to about 10 wt. %, insome embodiments from about 0.4 to about 5 wt. %, and in someembodiments, from about 0.5 to about 4 wt. % of the dispersion. Thedispersion may also contain one or more components to enhance theoverall properties of the resulting solid electrolyte. For example, thedispersion may contain a binder to further enhance the adhesive natureof the polymeric layer and also increase the stability of the particleswithin the dispersion. The binder may be organic in nature, such aspolyvinyl alcohols, polyvinyl pyrrolidones, polyvinyl chlorides,polyvinyl acetates, polyvinyl butyrates, polyacrylic acid esters,polyacrylic acid amides, polymethacrylic acid esters, polymethacrylicacid amides, polyacrylonitriles, styrene/acrylic acid ester, vinylacetate/acrylic acid ester and ethylene/vinyl acetate copolymers,polybutadienes, polyisoprenes, polystyrenes, polyethers, polyesters,polycarbonates, polyurethanes, polyamides, polyimides, polysulfones,melamine formaldehyde resins, epoxide resins, silicone resins orcelluloses. Crosslinking agents may also be employed to enhance theadhesion capacity of the binders. Such crosslinking agents may include,for instance, melamine compounds, masked isocyanates or crosslinkablepolymers, such as polyurethanes, polyacrylates or polyolefins, andsubsequent crosslinking. Dispersion agents may also be employed tofacilitate the ability to apply the layer to the anode. Suitabledispersion agents include solvents, such as aliphatic alcohols (e.g.,methanol, ethanol, i-propanol and butanol), aliphatic ketones (e.g.,acetone and methyl ethyl ketones), aliphatic carboxylic acid esters(e.g., ethyl acetate and butyl acetate), aromatic hydrocarbons (e.g.,toluene and xylene), aliphatic hydrocarbons (e.g., hexane, heptane andcyclohexane), chlorinated hydrocarbons (e.g., dichloromethane anddichloroethane), aliphatic nitriles (e.g., acetonitrile), aliphaticsulfoxides and sulfones (e.g., dimethyl sulfoxide and sulfolane),aliphatic carboxylic acid amides (e.g., methylacetamide,dimethylacetamide and dimethylformamide), aliphatic and araliphaticethers (e.g., diethylether and anisole), water, and mixtures of any ofthe foregoing solvents. A particularly suitable dispersion agent iswater.

In addition to those mentioned above, still other ingredients may alsobe used in the dispersion. For example, conventional fillers may be usedthat have a size of from about 10 nanometers to about 100 micrometers,in some embodiments from about 50 nanometers to about 50 micrometers,and in some embodiments, from about 100 nanometers to about 30micrometers. Examples of such fillers include calcium carbonate,silicates, silica, calcium or barium sulfate, aluminum hydroxide, glassfibers or bulbs, wood flour, cellulose powder carbon black, electricallyconductive polymers, etc. The fillers may be introduced into thedispersion in powder form, but may also be present in another form, suchas fibers.

Surface-active substances may also be employed in the dispersion, suchas ionic or non-ionic surfactants. Furthermore, adhesives may beemployed, such as organofunctional silanes or their hydrolysates, forexample 3-glycidoxypropyltrialkoxysilane, 3-aminopropyl-triethoxysilane,3-mercaptopropyl-trimethoxysilane, 3-metacryloxypropyltrimethoxysilane,vinyltrimethoxysilane or octyltriethoxysilane. The dispersion may alsocontain additives that increase conductivity, such as ethergroup-containing compounds (e.g., tetrahydrofuran), lactonegroup-containing compounds (e.g., γ-butyrolactone or γ-valerolactone),amide or lactam group-containing compounds (e.g., caprolactam,N-methylcaprolactam, N,N-dimethylacetamide, N-methylacetamide,N,N-dimethylformamide (DMF), N-methylformamide, N-methylformanilide,N-methylpyrrolidone (NMP), N-octylpyrrolidone, or pyrrolidone), sulfonesand sulfoxides (e.g., sulfolane (tetramethylenesulfone) ordimethylsulfoxide (DMSO)), sugar or sugar derivatives (e.g., saccharose,glucose, fructose, or lactose), sugar alcohols (e.g., sorbitol ormannitol), furan derivatives (e.g., 2-furancarboxylic acid or3-furancarboxylic acid), an alcohols (e.g., ethylene glycol, glycerol,di- or triethylene glycol).

The dispersion may be applied using a variety of known techniques, suchas by spin coating, impregnation, pouring, dropwise application,injection, spraying, doctor blading, brushing, printing (e.g., ink-jet,screen, or pad printing), or dipping. The viscosity of the dispersion istypically from about 0.1 to about 100,000 mPas (measured at a shear rateof 100 s⁻¹), in some embodiments from about 1 to about 10,000 mPas, insome embodiments from about 10 to about 1,500 mPas, and in someembodiments, from about 100 to about 1000 mPas.

If desired, a hydroxyl-functional nonionic polymer may also be employedin the outer layer(s) of the solid electrolyte. The term“hydroxy-functional” generally means that the compound contains at leastone hydroxyl functional group or is capable of possessing such afunctional group in the presence of a solvent. Without intending to belimited by theory, it is believed that the use of a hydroxy-functionalpolymer with a certain molecular weight can minimize the likelihood ofchemical decomposition at high voltages. For instance, the molecularweight of the hydroxy-functional polymer may be from about 100 to 10,000grams per mole, in some embodiments from about 200 to 2,000, in someembodiments from about 300 to about 1,200, and in some embodiments, fromabout 400 to about 800.

Any of a variety of hydroxy-functional nonionic polymers may generallybe employed for this purpose. In one embodiment, for example, thehydroxy-functional polymer is a polyalkylene ether. Polyalkylene ethersmay include polyalkylene glycols (e.g., polyethylene glycols,polypropylene glycols polytetramethylene glycols, polyepichlorohydrins,etc.), polyoxetanes, polyphenylene ethers, polyether ketones, and soforth. Polyalkylene ethers are typically predominantly linear, nonionicpolymers with terminal hydroxy groups. Particularly suitable arepolyethylene glycols, polypropylene glycols and polytetramethyleneglycols (polytetrahydrofurans), which are produced by polyaddition ofethylene oxide, propylene oxide or tetrahydrofuran onto water. Thepolyalkylene ethers may be prepared by polycondensation reactions fromdiols or polyols. The diol component may be selected, in particular,from saturated or unsaturated, branched or unbranched, aliphaticdihydroxy compounds containing 5 to 36 carbon atoms or aromaticdihydroxy compounds, such as, for example, pentane-1,5-diol,hexane-1,6-diol, neopentyl glycol, bis-(hydroxymethyl)-cyclohexanes,bisphenol A, dimer diols, hydrogenated dimer diols or even mixtures ofthe diols mentioned. In addition, polyhydric alcohols may also be usedin the polymerization reaction, including for example glycerol, di- andpolyglycerol, trimethylolpropane, pentaerythritol or sorbitol.

In addition to those noted above, other hydroxy-functional nonionicpolymers may also be employed in the present invention. Some examples ofsuch polymers include, for instance, ethoxylated alkylphenols;ethoxylated or propoxylated C₆-C₂₄ fatty alcohols; polyoxyethyleneglycol alkyl ethers having the general formula:CH₃—(CH₂)₁₀₋₁₆—(O—C₂H₄)₁₋₂₅—OH (e.g., octaethylene glycol monododecylether and pentaethylene glycol monododecyl ether); polyoxypropyleneglycol alkyl ethers having the general formula:CH₃—(CH₂)₁₀₋₁₆—(O—C₃H₆)₁₋₂₅—OH; polyoxyethylene glycol octylphenolethers having the following general formula:C₈H₁₇—(C₆H₄)—(O—C₂H₄)₁₋₂₅—OH (e.g., Triton™ X-100); polyoxyethyleneglycol alkylphenol ethers having the following general formula:C₉H₁₉—(C₆H₄)—(O—C₂H₄)₁₋₂₅—OH (e.g., nonoxynol-9); polyoxyethylene glycolesters of C₈-C₂₄ fatty acids, such as polyoxyethylene glycol sorbitanalkyl esters (e.g., polyoxyethylene (20) sorbitan monolaurate,polyoxyethylene (20) sorbitan monopalmitate, polyoxyethylene (20)sorbitan monostearate, polyoxyethylene (20) sorbitan monooleate, PEG-20methyl glucose distearate, PEG-20 methyl glucose sesquistearate, PEG-80castor oil, and PEG-20 castor oil, PEG-3 castor oil, PEG 600 dioleate,and PEG 400 dioleate) and polyoxyethylene glycerol alkyl esters (e.g.,polyoxyethylene-23 glycerol laurate and polyoxyethylene-20 glycerolstearate); polyoxyethylene glycol ethers of C₈-C₂₄ fatty acids (e.g.,polyoxyethylene-10 cetyl ether, polyoxyethylene-10 stearyl ether,polyoxyethylene-cetyl ether, polyoxyethylene-10 oleyl ether,polyoxyethylene-20 oleyl ether, polyoxyethylene-20 isohexadecyl ether,polyoxyethylene-15 tridecyl ether, and polyoxyethylene-6 tridecylether); block copolymers of polyethylene glycol and polypropylene glycol(e.g., Poloxamers); and so forth, as well as mixtures thereof.

The hydroxy-functional nonionic polymer may be incorporated into theouter layers in a variety of different ways. In certain embodiments, forinstance, the nonionic polymer may simply be incorporated into adispersion of conductive polymers. In such embodiments, theconcentration of the nonionic polymer in the layer may be from about 1wt. % to about 50 wt. %, in some embodiments from about 5 wt. % to about40 wt. %, and in some embodiments, from about 10 wt. % to about 30 wt.%. In other embodiments, however, the nonionic polymer may be appliedafter the initial outer layer(s) are formed. In such embodiments, thetechnique used to apply the nonionic polymer may vary. For example, thenonionic polymer may be applied in the form of a liquid solution usingvarious methods, such as immersion, dipping, pouring, dripping,injection, spraying, spreading, painting or printing, for example,inkjet, or screen printing. Solvents known to the person skilled in theart can be employed in the solution, such as water, alcohols, or amixture thereof. The concentration of the nonionic polymer in such asolution typically ranges from about 5 wt. % to about 95 wt. %, in someembodiments from about 10 wt. % to about 70 wt. %, and in someembodiments, from about 15 wt. % to about 50 wt. % of the solution. Ifdesired, such solutions may be generally free of conductive polymers.For example, conductive polymers may constitute about 2 wt. % or less,in some embodiments about 1 wt. % or less, and in some embodiments,about 0.5 wt. % or less of the solution.

D. External Polymer Coating

An external polymer coating may also be optionally employed thatoverlies the solid electrolyte. When employed, the external polymercoating typically contains one or more layers formed frompre-polymerized conductive polymer particles such as described above(e.g., dispersion of extrinsically conductive polymer particles). Theexternal coating may be able to further penetrate into the edge regionof the capacitor body to increase the adhesion to the dielectric andresult in a more mechanically robust part, which may reduce equivalentseries resistance and leakage current. Because it is generally intendedto improve the degree of edge coverage rather to impregnate the interiorof the anode body, the particles used in the external coating may have alarger size than those employed in the outer layers of the solidelectrolyte. For example, the ratio of the average size of the particlesemployed in the external polymer coating to the average size of theparticles employed in any dispersion of the solid electrolyte istypically from about 1.5 to about 30, in some embodiments from about 2to about 20, and in some embodiments, from about 5 to about 15. Forexample, the particles employed in the dispersion of the externalcoating may have an average size of from about 80 to about 500nanometers, in some embodiments from about 90 to about 250 nanometers,and in some embodiments, from about 100 to about 200 nanometers.

If desired, a crosslinking agent may also be employed in the externalpolymer coating to enhance the degree of adhesion to the solidelectrolyte. Typically, the crosslinking agent is applied prior toapplication of the dispersion used in the external coating. Suitablecrosslinking agents are described, for instance, in U.S. PatentPublication No. 2007/0064376 to Merker, et al. and include, forinstance, amines (e.g., diamines, triamines, oligomer amines,polyamines, etc.); polyvalent metal cations, such as salts or compoundsof Mg, Al, Ca, Fe, Cr, Mn, Ba, Ti, Co, Ni, Cu, Ru, Ce or Zn, phosphoniumcompounds, sulfonium compounds, etc. Particularly suitable examplesinclude, for instance, 1,4-diaminocyclohexane,1,4-bis(amino-methyl)cyclohexane, ethylenediamine, 1,6-hexanediamine,1,7-heptanediamine, 1,8-octanediamine, 1,9-nonanediamine,1,10-decanediamine, 1,12-dodecanediamine, N,N-dimethylethylenediamine,N,N,N′,N′-tetramethylethylenediamine,N,N,N′,N′-tetramethyl-1,4-butanediamine, etc., as well as mixturesthereof.

The crosslinking agent is typically applied from a solution ordispersion whose pH is from 1 to 10, in some embodiments from 2 to 7, insome embodiments, from 3 to 6, as determined at 25° C. Acidic compoundsmay be employed to help achieve the desired pH level. Examples ofsolvents or dispersants for the crosslinking agent include water ororganic solvents, such as alcohols, ketones, carboxylic esters, etc. Thecrosslinking agent may be applied to the capacitor body by any knownprocess, such as spin-coating, impregnation, casting, dropwiseapplication, spray application, vapor deposition, sputtering,sublimation, knife-coating, painting or printing, for example inkjet,screen or pad printing. Once applied, the crosslinking agent may bedried prior to application of the polymer dispersion. This process maythen be repeated until the desired thickness is achieved. For example,the total thickness of the entire external polymer coating, includingthe crosslinking agent and dispersion layers, may range from about 1 toabout 50 μm, in some embodiments from about 2 to about 40 μm, and insome embodiments, from about 5 to about 20 μm.

E. Other Optional Components

If desired, the capacitor element may also contain other layers as isknown in the art. For example, an adhesive layer may optionally beformed between the dielectric and solid electrolyte. The adhesive layermay, for instance, be present between the dielectric and pre-coat layerand/or between the pre-coat layer and the solid electrolyte. Regardless,the adhesive layer is typically formed from a relatively insulativeresinous material (natural or synthetic). Such materials may have aspecific resistivity of greater than about 10 Ω·cm, in some embodimentsgreater than about 100, in some embodiments greater than about 1,000Ω·cm, in some embodiments greater than about 1×10⁵ Ω·cm, and in someembodiments, greater than about 1×10¹⁰ Ω·cm. Some resinous materialsthat may be utilized in the present invention include, but are notlimited to, polyurethane, polystyrene, esters of unsaturated orsaturated fatty acids (e.g., glycerides), and so forth. For instance,suitable esters of fatty acids include, but are not limited to, estersof lauric acid, myristic acid, palmitic acid, stearic acid, eleostearicacid, oleic acid, linoleic acid, linolenic acid, aleuritic acid,shellolic acid, and so forth. These esters of fatty acids have beenfound particularly useful when used in relatively complex combinationsto form a “drying oil”, which allows the resulting film to rapidlypolymerize into a stable layer. Such drying oils may include mono-, di-,and/or tri-glycerides, which have a glycerol backbone with one, two, andthree, respectively, fatty acyl residues that are esterified. Forinstance, some suitable drying oils that may be used include, but arenot limited to, olive oil, linseed oil, castor oil, tung oil, soybeanoil, and shellac. These and other adhesive layer materials are describedin more detail U.S. Pat. No. 6,674,635 to Fife, et al.

If desired, the part may also be applied with a carbon layer (e.g.,graphite) and silver layer, respectively. The silver coating may, forinstance, act as a solderable conductor, contact layer, and/or chargecollector for the capacitor and the carbon coating may limit contact ofthe silver coating with the solid electrolyte. Such coatings may coversome or all of the solid electrolyte.

II. Terminations

Once formed, the capacitor element may be provided with terminations,particularly when employed in surface mounting applications. Forexample, the capacitor assembly may contain an anode termination towhich an anode lead of the capacitor element is electrically connectedand a cathode termination to which the cathode of the capacitor elementis electrically connected. Any conductive material may be employed toform the terminations, such as a conductive metal (e.g., copper, nickel,silver, nickel, zinc, tin, palladium, lead, copper, aluminum,molybdenum, titanium, iron, zirconium, magnesium, and alloys thereof).Particularly suitable conductive metals include, for instance, copper,copper alloys (e.g., copper-zirconium, copper-magnesium, copper-zinc, orcopper-iron), nickel, and nickel alloys (e.g., nickel-iron). Thethickness of the terminations is generally selected to minimize thethickness of the capacitor. For instance, the thickness of theterminations may range from about 0.05 to about 1 millimeter, in someembodiments from about 0.05 to about 0.5 millimeters, and from about0.07 to about 0.2 millimeters. One exemplary conductive material is acopper-iron alloy metal plate available from Wieland (Germany). Ifdesired, the surface of the terminations may be electroplated withnickel, silver, gold, tin, etc. as is known in the art to ensure thatthe final part is mountable to the circuit board. In one particularembodiment, both surfaces of the terminations are plated with nickel andsilver flashes, respectively, while the mounting surface is also platedwith a tin solder layer.

The terminations may be connected to the capacitor element using anytechnique known in the art. In one embodiment, for example, a lead framemay be provided that defines the cathode termination and anodetermination. To attach the electrolytic capacitor element to the leadframe, a conductive adhesive may initially be applied to a surface ofthe cathode termination. The conductive adhesive may include, forinstance, conductive metal particles contained with a resin composition.The metal particles may be silver, copper, gold, platinum, nickel, zinc,bismuth, etc. The resin composition may include a thermoset resin (e.g.,epoxy resin), curing agent (e.g., acid anhydride), and compound (e.g.,silane compounds). Suitable conductive adhesives may be described inU.S. Patent Application Publication No. 2006/0038304 to Osako, et al.Any of a variety of techniques may be used to apply the conductiveadhesive to the cathode termination. Printing techniques, for instance,may be employed due to their practical and cost-saving benefits. Theanode lead may also be electrically connected to the anode terminationusing any technique known in the art, such as mechanical welding, laserwelding, conductive adhesives, etc. Upon electrically connecting theanode lead to the anode termination, the conductive adhesive may then becured to ensure that the electrolytic capacitor element is adequatelyadhered to the cathode termination.

III. Housing

Due to the ability of the capacitor assembly to exhibit good electricalperformance in various environments, it is not necessary for thecapacitor element to be hermetically sealed within a housing.Nevertheless, in certain embodiments, it may be desired to hermeticallyseal the capacitor element within a housing. In one embodiment, forexample, the capacitor element may be hermetically sealed within ahousing in the presence of a gaseous atmosphere that contains an inertgas, thereby further limiting the amount of moisture supplied to thesolid electrolyte of the capacitor element.

The capacitor element may be sealed within a housing in various ways. Incertain embodiments, for instance, the capacitor element may be enclosedwithin a case, which may then be filled with a resinous material, suchas a thermoset resin (e.g., epoxy resin) that can be cured to form ahardened housing. Examples of such resins include, for instance, epoxyresins, polyimide resins, melamine resins, urea-formaldehyde resins,polyurethane resins, phenolic resins, polyester resins, etc. Epoxyresins are also particularly suitable. Still other additives may also beemployed, such as photoinitiators, viscosity modifiers, suspensionaiding agents, pigments, stress reducing agents, non-conductive fillers,stabilizers, etc. For example, the non-conductive fillers may includeinorganic oxide particles, such as silica, alumina, zirconia, magnesiumoxide, iron oxide, copper oxide, zeolites, silicates, clays (e.g.,smectite clay), etc., as well as composites (e.g., alumina-coated silicaparticles) and mixtures thereof. Regardless, the resinous material maysurround and encapsulate the capacitor element so that at least aportion of the anode and cathode terminations are exposed for mountingonto a circuit board. When encapsulated in this manner, the capacitorelement and resinous material form an integral capacitor assembly.

Of course, in alternative embodiments, it may be desirable to enclosethe capacitor element within a housing that remains separate anddistinct. In this manner, the atmosphere of the housing may be gaseousand contain at least one inert gas, such as nitrogen, helium, argon,xenon, neon, krypton, radon, and so forth, as well as mixtures thereof.Typically, inert gases constitute the majority of the atmosphere withinthe housing, such as from about 50 wt. % to 100 wt. %, in someembodiments from about 75 wt. % to 100 wt. %, and in some embodiments,from about 90 wt. % to about 99 wt. % of the atmosphere. If desired, arelatively small amount of non-inert gases may also be employed, such ascarbon dioxide, oxygen, water vapor, etc. In such cases, however, thenon-inert gases typically constitute 15 wt. % or less, in someembodiments 10 wt. % or less, in some embodiments about 5 wt. % or less,in some embodiments about 1 wt. % or less, and in some embodiments, fromabout 0.01 wt. % to about 1 wt. % of the atmosphere within the housing.

Any of a variety of different materials may be used to form the housing,such as metals, plastics, ceramics, and so forth. In one embodiment, forexample, the housing includes one or more layers of a metal, such astantalum, niobium, aluminum, nickel, hafnium, titanium, copper, silver,steel (e.g., stainless), alloys thereof (e.g., electrically conductiveoxides), composites thereof (e.g., metal coated with electricallyconductive oxide), and so forth. In another embodiment, the housing mayinclude one or more layers of a ceramic material, such as aluminumnitride, aluminum oxide, silicon oxide, magnesium oxide, calcium oxide,glass, etc., as well as combinations thereof.

The housing may have any desired shape, such as cylindrical, D-shaped,rectangular, triangular, prismatic, etc. Referring to FIG. 1, forexample, one embodiment of a capacitor assembly 100 is shown thatcontains a housing 122 and a capacitor element 120. In this particularembodiment, the housing 122 is generally rectangular. Typically, thehousing and the capacitor element have the same or similar shape so thatthe capacitor element can be readily accommodated within the interiorcavity. In the illustrated embodiment, for example, both the capacitorelement 120 and the housing 122 have a generally rectangular shape.

If desired, the capacitor assembly of the present invention may exhibita relatively high volumetric efficiency. To facilitate such highefficiency, the capacitor element typically occupies a substantialportion of the volume of an interior cavity of the housing. For example,the capacitor element may occupy about 30 vol. % or more, in someembodiments about 50 vol. % or more, in some embodiments about 60 vol. %or more, in some embodiments about 70 vol. % or more, in someembodiments from about 80 vol. % to about 98 vol. %, and in someembodiments, from about 85 vol. % to 97 vol. % of the interior cavity ofthe housing. To this end, the difference between the dimensions of thecapacitor element and those of the interior cavity defined by thehousing are typically relatively small.

Referring to FIG. 1, for example, the capacitor element 120 may have alength (excluding the length of the anode lead 6) that is relativelysimilar to the length of an interior cavity 126 defined by the housing122. For example, the ratio of the length of the anode to the length ofthe interior cavity ranges from about 0.40 to 1.00, in some embodimentsfrom about 0.50 to about 0.99, in some embodiments from about 0.60 toabout 0.99, and in some embodiments, from about 0.70 to about 0.98. Thecapacitor element 120 may have a length of from about 5 to about 10millimeters, and the interior cavity 126 may have a length of from about6 to about 15 millimeters. Similarly, the ratio of the height of thecapacitor element 120 (in the −z direction) to the height of theinterior cavity 126 may range from about 0.40 to 1.00, in someembodiments from about 0.50 to about 0.99, in some embodiments fromabout 0.60 to about 0.99, and in some embodiments, from about 0.70 toabout 0.98. The ratio of the width of the capacitor element 120 (in the−x direction) to the width of the interior cavity 126 may also rangefrom about 0.50 to 1.00, in some embodiments from about 0.60 to about0.99, in some embodiments from about 0.70 to about 0.99, in someembodiments from about 0.80 to about 0.98, and in some embodiments, fromabout 0.85 to about 0.95. For example, the width of the capacitorelement 120 may be from about 2 to about 7 millimeters and the width ofthe interior cavity 126 may be from about 3 to about 10 millimeters, andthe height of the capacitor element 120 may be from about 0.5 to about 2millimeters and the width of the interior cavity 126 may be from about0.7 to about 6 millimeters.

Although by no means required, the capacitor element may be attached tothe housing in such a manner that an anode termination and cathodetermination are formed external to the housing for subsequentintegration into a circuit. The particular configuration of theterminations may depend on the intended application. In one embodiment,for example, the capacitor assembly may be formed so that it is surfacemountable, and yet still mechanically robust. For example, the anodelead may be electrically connected to external, surface mountable anodeand cathode terminations (e.g., pads, sheets, plates, frames, etc.).Such terminations may extend through the housing to connect with thecapacitor. The thickness or height of the terminations is generallyselected to minimize the thickness of the capacitor assembly. Forinstance, the thickness of the terminations may range from about 0.05 toabout 1 millimeter, in some embodiments from about 0.05 to about 0.5millimeters, and from about 0.1 to about 0.2 millimeters. If desired,the surface of the terminations may be electroplated with nickel,silver, gold, tin, etc. as is known in the art to ensure that the finalpart is mountable to the circuit board. In one particular embodiment,the termination(s) are deposited with nickel and silver flashes,respectively, and the mounting surface is also plated with a tin solderlayer. In another embodiment, the termination(s) are deposited with thinouter metal layers (e.g., gold) onto a base metal layer (e.g., copperalloy) to further increase conductivity.

In certain embodiments, connective members may be employed within theinterior cavity of the housing to facilitate connection to theterminations in a mechanically stable manner. For example, referringagain to FIG. 1, the capacitor assembly 100 may include a connectionmember 162 that is formed from a first portion 167 and a second portion165. The connection member 162 may be formed from conductive materialssimilar to the external terminations. The first portion 167 and secondportion 165 may be integral or separate pieces that are connectedtogether, either directly or via an additional conductive element (e.g.,metal). In the illustrated embodiment, the second portion 165 isprovided in a plane that is generally parallel to a lateral direction inwhich the lead 6 extends (e.g., −y direction). The first portion 167 is“upstanding” in the sense that it is provided in a plane that isgenerally perpendicular the lateral direction in which the lead 6extends. In this manner, the first portion 167 can limit movement of thelead 6 in the horizontal direction to enhance surface contact andmechanical stability during use. If desired, an insulative material 7(e.g., Teflon™ washer) may be employed around the lead 6.

The first portion 167 may possess a mounting region (not shown) that isconnected to the anode lead 6. The region may have a “U-shape” forfurther enhancing surface contact and mechanical stability of the lead6. Connection of the region to the lead 6 may be accomplished using anyof a variety of known techniques, such as welding, laser welding,conductive adhesives, etc. In one particular embodiment, for example,the region is laser welded to the anode lead 6. Regardless of thetechnique chosen, however, the first portion 167 can hold the anode lead6 in substantial horizontal alignment to further enhance the dimensionalstability of the capacitor assembly 100.

Referring again to FIG. 1, one embodiment of the present invention isshown in which the connective member 162 and capacitor element 120 areconnected to the housing 122 through anode and cathode terminations 127and 129, respectively. More specifically, the housing 122 of thisembodiment includes an outer wall 123 and two opposing sidewalls 124between which a cavity 126 is formed that includes the capacitor element120. The outer wall 123 and sidewalls 124 may be formed from one or morelayers of a metal, plastic, or ceramic material such as described above.In this particular embodiment, the anode termination 127 contains afirst region 127 a that is positioned within the housing 122 andelectrically connected to the connection member 162 and a second region127 b that is positioned external to the housing 122 and provides amounting surface 201. Likewise, the cathode termination 129 contains afirst region 129 a that is positioned within the housing 122 andelectrically connected to the solid electrolyte of the capacitor element120 and a second region 129 b that is positioned external to the housing122 and provides a mounting surface 203. It should be understood thatthe entire portion of such regions need not be positioned within orexternal to the housing.

In the illustrated embodiment, a conductive trace 127 c extends in theouter wall 123 of the housing to connect the first region 127 a andsecond region 127 b. Similarly, a conductive trace 129 c extends in theouter wall 123 of the housing to connect the first region 127 a andsecond region 127 b. The conductive traces and/or regions of theterminations may be separate or integral. In addition to extendingthrough the outer wall of the housing, the traces may also be positionedat other locations, such as external to the outer wall. Of course, thepresent invention is by no means limited to the use of conductive tracesfor forming the desired terminations.

Regardless of the particular configuration employed, connection of theterminations 127 and 129 to the capacitor element 120 may be made usingany known technique, such as welding, laser welding, conductiveadhesives, etc. In one particular embodiment, for example, a conductiveadhesive 131 is used to connect the second portion 165 of the connectionmember 162 to the anode termination 127. Likewise, a conductive adhesive133 is used to connect the cathode of the capacitor element 120 to thecathode termination 129.

Optionally, a polymeric restraint may also be disposed in contact withone or more surfaces of the capacitor element, such as the rear surface,front surface, upper surface, lower surface, side surface(s), or anycombination thereof. The polymeric restraint can reduce the likelihoodof delamination by the capacitor element from the housing. In thisregard, the polymeric restraint may possesses a certain degree ofstrength that allows it to retain the capacitor element in a relativelyfixed positioned even when it is subjected to vibrational forces, yet isnot so strong that it cracks. For example, the restraint may possess atensile strength of from about 1 to about 150 Megapascals (“MPa”), insome embodiments from about 2 to about 100 MPa, in some embodiments fromabout 10 to about 80 MPa, and in some embodiments, from about 20 toabout 70 MPa, measured at a temperature of about 25° C. It is normallydesired that the restraint is not electrically conductive. Referringagain to FIG. 1, for instance, one embodiment is shown in which a singlepolymeric restraint 197 is disposed in contact with an upper surface 181and rear surface 177 of the capacitor element 120. While a singlerestraint is shown in FIG. 1, it should be understood that separaterestraints may be employed to accomplish the same function. In fact,more generally, any number of polymeric restraints may be employed tocontact any desired surface of the capacitor element. When multiplerestraints are employed, they may be in contact with each other orremain physically separated. For example, in one embodiment, a secondpolymeric restraint (not shown) may be employed that contacts the uppersurface 181 and front surface 179 of the capacitor element 120. Thefirst polymeric restraint 197 and the second polymeric restraint (notshown) may or may not be in contact with each other. In yet anotherembodiment, a polymeric restraint may also contact a lower surface 183and/or side surface(s) of the capacitor element 120, either inconjunction with or in lieu of other surfaces.

Regardless of how it is applied, it is typically desired that thepolymeric restraint is also in contact with at least one surface of thehousing to help further mechanically stabilize the capacitor elementagainst possible delamination. For example, the restraint may be incontact with an interior surface of one or more sidewall(s), outer wall,lid, etc. In FIG. 1, for example, the polymeric restraint 197 is incontact with an interior surface 107 of sidewall 124 and an interiorsurface 109 of outer wall 123. While in contact with the housing, it isnevertheless desired that at least a portion of the cavity defined bythe housing remains unoccupied to allow for the inert gas to flowthrough the cavity and limit contact of the solid electrolyte withoxygen. For example, at least about 5% of the cavity volume typicallyremains unoccupied by the capacitor element and polymer restraint, andin some embodiments, from about 10% to about 50% of the cavity volume.

Once connected in the desired manner, the resulting package ishermetically sealed as described above. Referring again to FIG. 1, forinstance, the housing 122 may also include a lid 125 that is placed onan upper surface of side walls 124 after the capacitor element 120 andthe polymer restraint 197 are positioned within the housing 122. The lid125 may be formed from a ceramic, metal (e.g., iron, copper, nickel,cobalt, etc., as well as alloys thereof), plastic, and so forth. Ifdesired, a sealing member 187 may be disposed between the lid 125 andthe side walls 124 to help provide a good seal. In one embodiment, forexample, the sealing member may include a glass-to-metal seal, Kovar®ring (Goodfellow Camridge, Ltd.), etc. The height of the side walls 124is generally such that the lid 125 does not contact any surface of thecapacitor element 120 so that it is not contaminated. The polymericrestraint 197 may or may not contact the lid 125. When placed in thedesired position, the lid 125 is hermetically sealed to the sidewalls124 using known techniques, such as welding (e.g., resistance welding,laser welding, etc.), soldering, etc. Hermetic sealing generally occursin the presence of inert gases as described above so that the resultingassembly is substantially free of reactive gases, such as oxygen orwater vapor.

It should be understood that the embodiments described are onlyexemplary, and that various other configurations may be employed in thepresent invention for hermetically sealing a capacitor element within ahousing. Referring to FIG. 2, for instance, another embodiment of acapacitor assembly 200 is shown that employs a housing 222 that includesan outer wall 123 and a lid 225 between which a cavity 126 is formedthat includes the capacitor element 120 and polymeric restraint 197. Thelid 225 includes an outer wall 223 that is integral with at least onesidewall 224. In the illustrated embodiment, for example, two opposingsidewalls 224 are shown in cross-section. The outer walls 223 and 123both extend in a lateral direction (−y direction) and are generallyparallel with each other and to the lateral direction of the anode lead6. The sidewall 224 extends from the outer wall 223 in a longitudinaldirection that is generally perpendicular to the outer wall 123. Adistal end 500 of the lid 225 is defined by the outer wall 223 and aproximal end 501 is defined by a lip 253 of the sidewall 224.

The lip 253 extends from the sidewall 224 in the lateral direction,which may be generally parallel to the lateral direction of the outerwall 123. The angle between the sidewall 224 and the lip 253 may vary,but is typically from about 60° to about 120°, in some embodiments fromabout 70° to about 110°, and in some embodiments, from about 80° toabout 100° (e.g., about 90°). The lip 253 also defines a peripheral edge251, which may be generally perpendicular to the lateral direction inwhich the lip 253 and outer wall 123 extend. The peripheral edge 251 islocated beyond the outer periphery of the sidewall 224 and may begenerally coplanar with an edge 151 of the outer wall 123. The lip 253may be sealed to the outer wall 123 using any known technique, such aswelding (e.g., resistance or laser), soldering, glue, etc. For example,in the illustrated embodiment, a sealing member 287 is employed (e.g.,glass-to-metal seal, Kovar® ring, etc.) between the components tofacilitate their attachment. Regardless, the use of a lip describedabove can enable a more stable connection between the components andimprove the seal and mechanical stability of the capacitor assembly.

Still other possible housing configurations may be employed in thepresent invention. For example, FIG. 3 shows a capacitor assembly 300having a housing configuration similar to that of FIG. 2, except thatterminal pins 327 b and 329 b are employed as the external terminationsfor the anode and cathode, respectively. More particularly, the terminalpin 327 a extends through a trace 327 c formed in the outer wall 323 andis connected to the anode lead 6 using known techniques (e.g., welding).An additional section 327 a may be employed to secure the pin 327 b.Likewise, the terminal pin 329 b extends through a trace 329 c formed inthe outer wall 323 and is connected to the cathode via a conductiveadhesive 133 as described above.

The embodiments shown in FIGS. 1-3 are discussed herein in terms of onlya single capacitor element. It should also be understood, however, thatmultiple capacitor elements may also be hermetically sealed within ahousing. The multiple capacitor elements may be attached to the housingusing any of a variety of different techniques. Referring to FIG. 4, forexample one particular embodiment of a capacitor assembly 400 thatcontains two capacitor elements is shown and will now be described inmore detail. More particularly, the capacitor assembly 400 includes afirst capacitor element 420 a in electrical communication with a secondcapacitor element 420 b. In this embodiment, the capacitor elements arealigned so that their major surfaces are in a horizontal configuration.That is, a major surface of the capacitor element 420 a defined by itswidth (−x direction) and length (−y direction) is positioned adjacent toa corresponding major surface of the capacitor element 420 b. Thus, themajor surfaces are generally coplanar. Alternatively, the capacitorelements may be arranged so that their major surfaces are not coplanar,but perpendicular to each other in a certain direction, such as the −zdirection or the −x direction. Of course, the capacitor elements neednot extend in the same direction.

The capacitor elements 420 a and 420 b are positioned within a housing422 that contains an outer wall 423 and sidewalls 424 and 425 thattogether define a cavity 426. Although not shown, a lid may be employedthat covers the upper surfaces of the sidewalls 424 and 425 and sealsthe assembly 400 as described above. Optionally, a polymeric restraintmay also be employed to help limit the vibration of the capacitorelements. In FIG. 4, for example, separate polymer restraints 497 a and497 b are positioned adjacent to and in contact with the capacitorelements 420 a and 420 b, respectively. The polymer restraints 497 a and497 b may be positioned in a variety of different locations. Further,one of the restraints may be eliminated, or additional restraints may beemployed. In certain embodiments, for example, it may be desired toemploy a polymeric restraint between the capacitor elements to furtherimprove mechanical stability.

In addition to the capacitor elements, the capacitor assembly alsocontains an anode termination to which anode leads of respectivecapacitor elements are electrically connected and a cathode terminationto which the cathodes of respective capacitor elements are electricallyconnected. Referring again to FIG. 4, for example, the capacitorelements are shown connected in parallel to a common cathode termination429. In this particular embodiment, the cathode termination 429 isinitially provided in a plane that is generally parallel to the bottomsurface of the capacitor elements and may be in electrical contact withconductive traces (not shown). The capacitor assembly 400 also includesconnective members 427 and 527 that are connected to anode leads 407 aand 407 b, respectively, of the capacitor elements 420 a and 420 b. Moreparticularly, the connective member 427 contains an upstanding portion465 and a planar portion 463 that is in connection with an anodetermination (not shown). Likewise, the connective 527 contains anupstanding portion 565 and a planar portion 563 that is in connectionwith an anode termination (not shown). Of course, it should beunderstood that a wide variety of other types of connection mechanismsmay also be employed.

Regardless of its particular construction, the resulting capacitorassembly can exhibit a variety of beneficial properties. For example,the capacitor assembly can exhibit a high percentage of its wetcapacitance, which enables it to have only a small capacitance lossand/or fluctuation in the presence of atmosphere humidity. Thisperformance characteristic is quantified by the “wet-to-dry capacitancepercentage”, which is determined by the equation:Wet-to-Dry Capacitance=(Dry Capacitance/Wet Capacitance)×100

The capacitor assembly may exhibit a wet-to-dry capacitance percentageof about 50% or more, in some embodiments about 60% or more, in someembodiments about 70% or more, and in some embodiments, from about 80%to 100%. The dry capacitance may be about 30 nanoFarads per squarecentimeter (“nF/cm²”) or more, in some embodiments about 100 nF/cm² ormore, in some embodiments from about 200 to about 3,000 nF/cm², and insome embodiments, from about 400 to about 2,000 nF/cm², measured at afrequency of 120 Hz.

The capacitor may also exhibit a relatively low equivalence seriesresistance (“ESR”), such as about 200 mohms, in some embodiments lessthan about 150 mohms, and in some embodiments, from about 0.1 to about100 mohms, measured at an operating frequency of 100 kHz. It is alsobelieved that the dissipation factor of the capacitor assembly may bemaintained at relatively low levels. The dissipation factor generallyrefers to losses that occur in the capacitor and is usually expressed asa percentage of the ideal capacitor performance. For example, thedissipation factor of the capacitor of the present invention istypically from about 0.5% to about 25%, in some embodiments from about0.8% to about 10%, and in some embodiments, from about 1% to about 5%,as determined at a frequency of 120 Hz. The capacitor assembly may alsobe able to be employed in high voltage applications, such as at ratedvoltages of about 16 volts or more, in some embodiments about 25 voltsor more, and in some embodiments, from about volts to about 200 volts.

The present invention may be better understood by reference to thefollowing examples.

Test Procedures

Equivalent Series Resistance (ESR)

Equivalence series resistance may be measured using a Keithley 3330Precision LCZ meter with Kelvin Leads 2.2 volt DC bias and a 0.5 voltpeak to peak sinusoidal signal. The operating frequency may 100 kHz andthe temperature may be 23° C. 2° C.

Dissipation Factor

The dissipation factor may be measured using a Keithley 3330 PrecisionLCZ meter with Kelvin Leads with 2.2 volt DC bias and a 0.5 volt peak topeak sinusoidal signal. The operating frequency may be 120 Hz and thetemperature may be 23° C. 2° C.

Capacitance

The capacitance may be measured using a Keithley 3330 Precision LCZmeter with Kelvin Leads with 2.2 volt DC bias and a 0.5 volt peak topeak sinusoidal signal. The operating frequency may be 120 Hz and thetemperature may be 23° C. 2° C. In some cases, the “wet-to-dry”capacitance can be determined. The “dry capacitance” refers to thecapacitance of the part before application of the solid electrolyte,graphite, and silver layers, while the “wet capacitance” refers to thecapacitance of the part after formation of the dielectric, measured in14% nitric acid in reference to 1 mF tantalum cathode with 10 volt DCbias and a 0.5 volt peak to peak sinusoidal signal after 30 seconds ofelectrolyte soaking.

Leakage Current

Leakage current may be measured using a leakage test meter at atemperature of 23° C. 2° C. and at the rated voltage (e.g., 16 volts)after a minimum of 60 seconds (e.g., 90 seconds, 180 seconds, 300seconds).

Temperature Life Testing

Temperature life testing may be conducted at a temperature of 105° C.and rated voltage (e.g., 16 volts). The recovery time after the testconditions may be 30 minutes. The ESR and leakage current may bemeasured at times of 1,500 hours and 3,000 hours after the recoverytime.

Example 1

70,000 μFV/g tantalum powder was used to form anode samples. Each anodesample was embedded with a tantalum wire, sintered at 1380° C., andpressed to a density of 5.8 g/cm³. The resulting pellets had a size of4.10×2.75×0.60 mm. The pellets were anodized to 42.0 volts inwater/phosphoric acid electrolyte with a conductivity of 8.6 mS at atemperature of 85° C. to form the dielectric layer. The pellets wereanodized again to 90 volts in a water/boric acid/disodium tetraboratewith a conductivity of 2.0 mS at a temperature of 30° C. for 25 secondsto form a thicker oxide layer built up on the outside.

A conductive polymer coating was then formed by dipping the anode into abutanol solution of iron (III) toluenesulfonate (Clevios™ C, H. C.Starck) for 5 minutes and consequently into 3,4-ethylenedioxythiophene(Clevios™ M, H. C. Starck) for 1 minute. After 45 minutes ofpolymerization, a thin layer of poly(3,4-ethylenedioxythiophene) wasformed on the surface of the dielectric. The anode was washed inmethanol to remove reaction by-products, anodized in a liquidelectrolyte, and washed again in methanol. This process was repeated 6times. Thereafter, the parts were dipped into a dispersedpoly(3,4-ethylenedioxythiophene) having a solids content 2.0% andviscosity 20 mPa·s (Clevios™ K, Heraeus). Upon coating, the parts weredried at 125° C. for 20 minutes. This process was repeated 3 times.Thereafter, the parts were dipped into a dispersedpoly(3,4-ethylenedioxythiophene) having a solids content of 2% andviscosity 160 mPa·s (Clevios™ K, Heraeus). Upon coating, the parts weredried at 125° C. for 20 minutes. This process was repeated 8 times. Theparts were then dipped into a graphite dispersion and dried. Finally,the parts were dipped into a silver dispersion and dried. Multiple parts(4000) of 47 μF/16V capacitors were made in this manner and encapsulatedin a silica resin.

Example 2

Capacitors were formed in the manner described in Example 1, except thattwo (2) pre-coat layers of an organometallic compound were used thatcontained a solution of γ-glycidoxypropyl-trimethoxysilane in ethanol(1.0%). Multiple parts (4000) of 47 μF/16V capacitors were formed andencapsulated in a silica resin.

Example 3

Capacitors were formed in the manner described in Example 1, except thatthree (3) pre-coat layers of an organometallic compound were used thatcontained a solution of γ-glycidoxypropyl-trimethoxysilane in ethanol(1.0%). Multiple parts (4000) of 47 μF/16V capacitors were formed andencapsulated in a silica resin.

Example 4

Capacitors were formed in the manner described in Example 1, except thatfour (4) pre-coat layers of an organometallic compound were used thatcontained a solution of γ-glycidoxypropyl-trimethoxysilane in ethanol(1.0%). Multiple parts (4000) of 47 μF/16V capacitors were formed andencapsulated in a silica resin.

The finished capacitors of Examples 1-4 were then tested for electricalperformance. The median results of capacitance (CAP), “wet-to-dry”capacitance (W/D), dissipation factor (Df), ESR and leakage current(DCL) are set forth below in Table 1. The median results of ESR changeand leakage current within 105° C. life testing at rated voltage are setforth below in Table 2.

TABLE 1 Electrical Properties 1^(st) 2^(nd) DCL @ DCL @ CAP W/D Df ESR90 sec 180 sec [μF] [%] [%] [mohms] [μA] [μA] Example 1 47.82 91.8 1.440.5 156.80 39.17 Example 2 45.73 87.7 1.2 30.7 55.38 14.24 Example 344.11 84.6 1.2 31.4 4.64 1.67 Example 4 43.01 82.5 1.0 30.2 3.33 1.13

TABLE 2 Temperature Life Testing Results Time ESR change DCL @ 300 sec[h] [%] [μA] Example 1 0 — 7.31 1500 12.2 98.82 3000 18.5 97.90 Example4 0 — 0.34 1500 17.4 1.20 3000 19.7 0.96

These and other modifications and variations of the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention. Inaddition, it should be understood that aspects of the variousembodiments may be interchanged both in whole or in part. Furthermore,those of ordinary skill in the art will appreciate that the foregoingdescription is by way of example only, and is not intended to limit theinvention so further described in such appended claims.

What is claimed is:
 1. A capacitor assembly comprising a capacitorelement, the capacitor element comprising: a sintered porous anode body;a dielectric that overlies the anode body; a pre-coat layer thatoverlies the dielectric that is formed from an organometallic compound;a solid electrolyte that overlies the pre-coat layer, wherein the solidelectrolyte contains an inner layer and an outer layer, wherein theinner layer is formed from an in situ-polymerized conductive polymer andthe outer layer is formed from pre-polymerized conductive polymerparticles, and wherein the inner layer is formed with an oxidativecatalyst that includes transition metal cations.
 2. The capacitorassembly of claim 1, wherein the anode body includes tantalum and thedielectric includes tantalum pentoxide.
 3. The capacitor assembly ofclaim 1, wherein the organometallic compound has the following generalformula:

wherein, M is an organometallic atom; R₁, R₂, and R₃ are independentlyan alkyl or a hydroxyalkyl, wherein at least one of R₁, R₂, and R₃ is ahydroxyalkyl; n is an integer from 0 to 8; X is an organic or inorganicfunctional group.
 4. The capacitor assembly of claim 3, wherein M issilicon.
 5. The capacitor assembly of claim 4, wherein the hydroxyalkylis OCH₃.
 6. The capacitor assembly of claim 3, wherein R₁, R₂, and R₃are a hydroxyalkyl.
 7. The capacitor assembly of claim 1, wherein theorganometallic compound is 3-aminopropyltrimethoxysilane,3-aminopropyltriethoxysilane, 3-aminopropylmethyldimethoxysilane,3-aminopropylmethyldiethoxysilane,3-(2-aminoethyl)aminopropyltrimethoxysilane,3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane,3-mercaptopropylmethyldimethoxysilane,3-mercaptopropylmethyldiethoxysilane, glycidoxymethyltrimethoxysilane,glycidoxymethyltriethoxysilane, glycidoxymethyl-tripropoxysilane,glycidoxymethyltributoxysilane, β-glycidoxyethyltrimethoxysilane,β-glycidoxyethyltriethoxysilane, β-glycidoxyethyl-tripropoxysilane,β-glycidoxyethyl-tributoxysilane, β-glycidoxyethyltrimethoxysilane,α-glycidoxyethyltriethoxysilane, α-glycidoxyethyltripropoxysilane,α-glycidoxyethyltributoxysilane, γ-glycidoxypropyl-trimethoxysilane,γ-glycidoxypropyltriethoxysilane, γ-glycidoxypropyl-tripropoxysilane,γ-glycidoxypropyltributoxysilane, β-glycidoxypropyltrimethoxysilane,β-glycidoxypropyl-triethoxysilane, β-glycidoxypropyltripropoxysilane,α-glycidoxypropyltributoxysilane, α-glycidoxypropyltrimethoxysilane,α-glycidoxypropyltriethoxysilane, α-glycidoxypropyl-tripropoxysilane,α-glycidoxypropyltributoxysilane, γ-glycidoxybutyltrimethoxysilane,δ-glycidoxybutyltriethoxysilane, δ-glycidoxybutyltripropoxysilane,δ-glycidoxybutyl-tributoxysilane, δ-glycidoxybutyltrimethoxysilane,γ-glycidoxybutyltriethoxysilane, γ-glycidoxybutyltripropoxysilane,γ-propoxybutyltributoxysilane, δ-glycidoxybutyl-trimethoxysilane,δ-glycidoxybutyltriethoxysilane, δ-glycidoxybutyltripropoxysilane,α-glycidoxybutyltrimethoxysilane, α-glycidoxybutyltriethoxysilane,α-glycidoxybutyl-tripropoxysilane, α-glycidoxybutyltributoxysilane, or acombination thereof.
 8. The capacitor assembly of claim 1, wherein theinner layer is formed by 3,4-ethylenedioxthiophene or a derivativethereof, with an oxidative catalyst, wherein the catalyst includes aniron (III) salt of an aromatic sulfonic acid.
 9. The capacitor assemblyof claim 1, wherein the solid electrolyte contains from 2 to 30 innerlayers that are formed from an in situ-polymerized conductive polymer.10. The capacitor assembly of claim 1, wherein the outer layer is formedfrom a dispersion of particles that contain a polymeric counterion andan extrinsically conductive polymer, wherein the extrinsicallyconductive polymer is poly(3,4-ethylenedioxythiophene) or a derivativethereof.
 11. The capacitor assembly of claim 1, wherein the outer layeris formed from an intrinsically conductive polymer having repeatingunits of the following formula:

wherein, R is (CH₂)_(a)—O—(CH₂)_(b); a is from 0 to 10; b is from 1 to18; Z is an anion; X is a cation.
 12. The capacitor assembly of claim 1,wherein at least a portion of the pre-polymerized conductive polymerparticles in the solid electrolyte have an average size of from about 1to about 80 nanometers.
 13. The capacitor assembly of claim 1, whereinthe outer layer is generally free of in situ-polymerized conductivepolymers.
 14. The capacitor assembly of claim 1, wherein the solidelectrolyte contains from 2 to 30 outer layers that are formed frompre-polymerized conductive polymer particles.
 15. The capacitor assemblyof claim 1, further comprising an external polymer coating that overliesthe solid electrolyte and contains pre-polymerized conductive polymerparticles and a cross-linking agent.
 16. The capacitor assembly of claim15, wherein at least a portion of the conductive polymer particles inthe external polymer coating have an average size of from about 80 toabout 500 nanometers.
 17. The capacitor assembly of claim 1, furthercomprising an anode termination that is in electrical connection withthe anode body; a cathode termination that is in electrical connectionwith the solid electrolyte; and a housing that encloses the capacitorelement and leaves exposed at least a portion of the anode terminationand the cathode termination.
 18. The capacitor assembly of claim 17,wherein the housing is formed from a resinous material that encapsulatesthe capacitor element.
 19. The capacitor assembly of claim 17, whereinthe housing defines an interior cavity within which the capacitorelement is positioned, wherein the interior cavity has a gaseousatmosphere that includes an inert gas.
 20. The capacitor assembly ofclaim 1, wherein after being subjected to an applied voltage for 90seconds, the assembly exhibits a leakage current of about 50 microampsor less.
 21. The capacitor assembly of claim 1, wherein after beingsubjected to an applied voltage for 180 seconds, the assembly exhibits aleakage current that is greater than 50% of the leakage currentexhibited after being subjected to the applied voltage for 90 seconds.22. The capacitor assembly of claim 21, wherein the assembly exhibits aleakage current of about 20 microamps or less after being subjected tothe applied voltage for 180 seconds.
 23. A method for forming acapacitor assembly, the method comprising: anodically oxidizing asintered porous anode body to form an anode; applying a solution to theanode that includes an organometallic compound having the followinggeneral formula:

wherein, M is an organometallic atom; r₁, r₂, and r₃ are independentlyan alkyl or a hydroxyalkyl, wherein at least one of R₁, R₂, and R₃ is ahydroxyalkyl; n is an integer from 0 to 8; X is an organic or inorganicfunctional group; polymerizing a conductive polymer precursor monomer inthe presence of an oxidative catalyst that includes transition metalcations to form an inner solid electrolyte layer that overlies thepre-coat layer; and applying a dispersion of conductive polymerparticles to form an outer solid electrolyte layer that overlies theinner solid electrolyte layer.
 24. The method of claim 23, wherein thesolution includes an organic solvent.
 25. The method of claim 24,further comprising drying the solution after application to the anode.26. The method of claim 23, wherein the precursor monomer and oxidativecatalyst are sequentially applied.
 27. The method of claim 23, whereinat least a portion of the conductive polymer particles have an averagesize of from about 1 to about 80 nanometers.